I have a large collection of Ghostly pets. The Ghost Boar with Armor Barding will most likely going away with WoD. He's a tame that maybe you shouldn't be able to tame. Blizzard has patched tames like this in the past. For instance, you can no longer tame the Boar on Fire with Armor Barding. The Slime or Three-headed Ghost Hydra. The summoned Spore. The Ghost Wolf. The list goes on. You'll want to grab this and it's fairly simple.
First head to the bottom of the Southern Barrens. Because of the disruption to the world, you'll want to then head west. Pass over Razorfen Downs. You want to go farther west to Razorfen Kraul. It is to the west of the road.
Side Note: Traveling to that part of the world is easy for horde with two main cities to choose from. Alliance engineers can use their portal tool to get to Gadgetzan. Or they can ask a mage to port them to the obliterated city of Theramore. Otherwise a boat trip to Dustwallow Marsh or Ratchet will have to do.
Once you're in the instance, you should remove your weapon, legendary cloak (if you have one), and any AoE proc trinkets. You can punch the mobs to death, or just run through them and FD.
You'll make your way, bending right, and then walking up a little crevase (passing the boss you want to get to on your left). Make a hard left and you'll see him in a little hut. I punched out everyone on my way there. Don't get punch-crazy and nuke the boss too.
Get your tame button ready. Then walk over to the boss to engage him. Don't hit him. Stand around and wait for him to cast a summoning spell for your new pet. Once it's arrived, tame it. It may die afterwards. That's okay. Just rez him and he'll still be ghostly.